The Golden rule of Compression (above all others) is this:

Try to compress your video only once. Each time you compress your product, you're taking a chance and lowering it's final quality.

I keep seeing people use tools like Handbrake to rip a DVD into MPG4 and then use another tool, like Apple Compressor to make ProRes. That's taking a distribution quality codec (MPEG2) taking it to another distribution quality (h.264) and then to an editorial codec.

Try, when possible to only transform your video once - at the end of your editorial process.